Give the common elements from both datasets.

To find the common elements between two datasets, convert one dataset into a hash set for O(1) lookups, then iterate over the second dataset and collect elements that exist in the set. This approach runs in O(n + m) time and O(min(n, m)) additional space, where n and m are the sizes of the two datasets. For example, if dataset A = [1,2,3,4] and dataset B = [3,4,5,6], the common elements are [3,4].